1. Field of the Invention
The present invention relates to a method of electron-beam exposure and a mask as well as an electron-beam exposure system used therein, and more particularly to an electron-beam exposure method of segmented mask-pattern transfer type that is employed to manufacture a semiconductor device and especially suited for the proximity effect correction and a mask as well as an electron-beam exposure system used therein.
2. Description of the Related Art
In the electron-beam exposure that is performed in the step of lithography for manufacturing a semiconductor device, the proximity effect caused by scattered electrons within a substrate and its coating resist layer strongly affects the linewidth accuracy of projection patterns. For instance, in closely spaced line and space patterns, electrons that enter into an exposed section may be severely scattered (back-scattering) within a substrate, and a resist in an adjacent unexposed section may be subjected to exposure (background exposure) by such back-scattering electrons. As a result, edge sections and central section of one pattern become displaying different distributions of deposited energy, as shown in FIG. 4, and a prescribed pattern that is set at an appropriate threshold level of energy becomes unobtainable when the resist is developed (particularly in edge sections). This highlights the fact that the proximity effect correction is one of the essential techniques in the art.
As the actual method of the proximity effect correction, there are known the dose compensation method in which, at the time of the pattern exposure, the optimum dose is appropriately chosen depending on the dose of background exposure and the GHOST exposure method wherein correction exposure is made so as to bring the dose of the background exposure to a constant level in all regions where pattern exposure is carried out.
In the cell projection method and the variable-shaped beam exposure method both of which are currently widely used methods of electron-beam exposure, in order to make the proximity effect correction according to the dose compensation method, the self-consistent method using the exposure intensity distribution (EID) function, the pattern density method or the like has been presently employed, any of which requires complicated calculations. In consequence, a lengthy time is required for the data processing, and besides for every different pattern to transfer, another set of complicated calculations of this sort must be made.
The GHOST method is a technique in which, after the primary pattern (the positive pattern) for exposure is subjected to exposure, weak correction exposure (GHOST exposure) is performed with the beam that is formed by defocusing the inverse pattern of the positive pattern over the back-scattering range, and thereby the proximity effect that may be brought about through back-scattering of the incident electrons for the positive pattern exposure is corrected. FIG. 5 is a diagram in explaining the principle of proximity effect correction according to the offset GHOST method that is a sort of the GHOST method, which shows schematically the distribution of deposited energy by the electron-beam exposure. FIG. 5(a) presents the distribution of deposited energy with the primary pattern of line and space (1/1) and FIG. 5(b), the distribution of deposited energy by the correction exposure with the beam that is formed by defocusing the inverse pattern over the back-scattering range, while FIG. 5(c) illustrates the distribution of deposited energy in the case the correction exposure to provide such a distribution of deposited energy as shown in FIG. 5(b) is applied to the exposed region of FIG. 5(a). In the drawings, the energy of forward-scattering electrons is set to be 1, and xcex7 and xcex2b represent the back-scattering coefficient and the back-scattering range, respectively. By making the proximity effect correction according to the GHOST method, the dose of background exposure can be brought to a constant level as shown in FIG. 5(c). Consequently, the distribution of deposited energy can become uniform throughout and the linewidth accuracy of the pattern, improved.
However, to apply the GHOST method of this sort to the cell projection lithography method or the variable-shaped beam exposure method, the exposure intensity must be none the less calculated using the EID function or the like. In addition, since complicated calculations are necessary for formation of the inverse pattern, considerable time is required for data processing. The projection of the inverse pattern obtained in this way also takes time. These factors all contribute to marked reduction of the throughput
Meanwhile, as a novel method of electron-beam exposure to replace the cell projection lithography method and the variable-shaped beam exposure method, an electron-beam exposure method of segmented mask-pattern transfer type has been recently proposed. This electron-beam exposure method of segmented mask-pattern transfer type is a method wherein a prescribed primary pattern for exposure is segmented into a plurality of divisions and every said divisions is subjected to exposure one by one till the whole of this prescribed primary pattern is transferred. Although the prescribed primary pattern is segmented into a plurality of divisions, this electron-beam exposure method of segmented mask-pattern transfer type uses a mask onto which the whole segmented portions of the prescribed pattern of one chip are formed in all. In this respect, the electron-beam exposure method of segmented mask-pattern transfer type is altogether different from the variable-shaped beam exposure method wherein a pattern that is to be formed is not actually formed onto the mask but processed as soft data or the cell projection lithography method which employs a mask onto which only repeated parts of a prescribed pattern is formed.
This electron-beam exposure method of segmented mask-pattern transfer type is explained well in the section of the prior art in Japanese Patent Application Laid-out No. 176720/1999 with reference to FIG. 2 in the publication. On the basis of this description, the electron-beam exposure method of segmented mask-pattern transfer type is described below.
FIG. 6 is a schematic view in explaining the electron-beam exposure method of segmented mask-pattern transfer type. In FIG. 6, referential numeral 100 indicates a mask; 100a, a division on the mask; 100b, a demarcation region between divisions 100a; 110, a substrate coated with a resist, such as a wafer; 110a, a region for one die (one chip) on the substrate 110; 110b, a region for projection on the substrate 110, each corresponding to a division 100a; AX, an optical axis of an optical system of charged particle beam; EB, a charged particle beam and CO, a crossover point of the optical system of charged particle beam.
On the mask 100, being separated by a demarcation region 100b without a pattern, there are present numerous divisions 100a each of which is provided, on a membrane, a pattern to be transferred onto the substrate 110. Further, a support structure in the form of a grid is set over the demarcation region 100b, protecting the membrane thermally and mechanically. The mask 100 herein is a scattering membrane mask wherein, on a membrane, for example, a silicon nitride film with a thickness of 100 nm or so, there are formed electron-beam scatterer patterns made of, for example, tungsten with a thickness of 50 nm or so. This scattering membrane mask is the mask used mainly for the electron-beam exposure method of scattering-angle limiting type (referred to as xe2x80x9cSAL typexe2x80x9d hereinafter) and the exposure method herein is assumed to be the SAL type.
Every division 100a is provided with one of segmented patterns which the pattern that is to be transferred onto a region 110a for one die on the substrate 110 is segmented into, and every segmented pattern is transferred onto the substrate 110, one by one. The external appearance of the substrate 110 is as shown in FIG. 6(b). A section (the Va section of FIG. 6(b)) of the substrate 110 is shown in FIG. 6(a) on an enlarged scale.
In FIG. 6, the z-axis is taken parallel to the optical axis AX of the optical system of charged particle beam, and the x-axis and y-axis are taken parallel to the directions of the array of divisions 100a, respectively. While the mask 100 and the substrate 110 are moved continuously in opposite directions along the x-axis as arrows Fm and Fw indicate, respectively, patterns of divisions 100a in one line are transferred in succession through step-by-step scanning of the charged particle beam in the direction of the y-axis. After completing projection of the patterns in one line, divisions 100a in the next of that line in the direction of the x-axis receive scanning of the charged particle beam. Thereafter, in the same manner, projection (segmented projection) of divisions. 100a is successively performed one by one so as to transfer the whole pattern for one die (chip).
The scanning order over the divisions 100a and the transcribing order onto the substrate 110 are presented by lines with arrowheads, Am and Aw, respectively. Hereat, the directions of movements for the mask 100 and the substrate 110 are opposite to each other, because the x-axis and y-axis for the mask 100 and the substrate 110 are reversed by a pair of projection lenses, respectively.
When the projection (segmented projection) is carried out in this manner, if patterns of divisions 100a in one line lying in the direction of the y-axis are projected on the substrate 110 by a pair of projection lenses as they are, gaps corresponding to the demarcation region 100b develop between regions for projection 110b on the substrate 110, each region for projection corresponding to a division 100a, respectively. To overcome this problem, the charged particle beam EB having passed through each division 110a is made deflected as much as the width Ly of the demarcation region 100b in the direction of the y-axis, whereby correction for the pattern projection position is made.
For the direction of the x-axis, besides moving the transmittable scattering mask 100 and the substrate 110 at respective specific speeds, in proportion to the ratio of pattern reduction, similar care is also taken. That is, when completing projection of divisions 100a in one line and turning to projection of divisions 100a in the next line, the charged particle beam EB is made deflected as much as the width Lx of the demarcation region 100b in the direction of the x-axis, whereby correction for the pattern projection position is made so as not to create a gap in the direction of the x-axis between regions for projection 110b. 
As described above, in the segmented mask-pattern transfer type method, a mask onto which the whole segmented portions of the prescribed pattern of one chip are formed in all is used so that the throughput thereof can be markedly improved as compared with the conventional cell projection lithography method and the variable-shaped beam exposure method.
Further, in the segmented mask-pattern transfer type method, since a support structure in the form of a grid can be set over the demarcation region 100b which is formed between respective divisions 100a, bending and thermal distortion of the mask substrate which may result from irradiation of the charged particle beam can be suppressed well and exposure projection can be performed with high accuracy.
In the electron-beam exposure method of segmented mask-pattern transfer type described above, correction of the afore-mentioned proximity effect is still a matter of great importance.
As a proximity effect correction method for the electron-beam exposure method of segmented mask-pattern transfer type, G. P. Watson and others (J.Vac.Sci.Technol. B13(6), 2504-2507 (1995)) proposed a proximity effect correction method according to the SCALPEL (registered trademark) GHOST method in which the afore-mentioned GHOST is applied to the SAL type electron-beam exposure method.
The proximity effect correction method by G. P. Watson and others is described below.
With respect to a mask for this SAL type electron-beam exposure method, there is used a mask (referred to as a xe2x80x9cscattering membrane maskxe2x80x9d, hereinafter) in which a apattern made of an electron-beam scatterer, for example, tungsten with a thickness of 50 nm or so, is formed on an electron-beam transmittable membrane (referred to simply as a xe2x80x9cmembranexe2x80x9d, hereinafter) with a relatively small electron-beam scattering power, for example, a silicon nitride film with a thickness of 100 nm or so. The exposure is carried out over a wafer by an electron beam consisting of electrons which are not scattered or scattered only with relatively small scattering angles, having transmitted the membrane region where no scatterer is formed. Meanwhile, electrons scattered with large scattering angles, having transmitted the scatterer region, are cut off by a limiting aperture section disposed in the position or the vicinity of the cross-over. In this way, the image contrast is formed on the wafer through the difference of the electron-beam scattering between the membrane region and the scatterer region.
Although, in the mask in the above description of the segmented mask-pattern transfer type method, the demarcation region to partition prescribed patterns is in the form of a grid, it can be stripe-shaped and a mask used herein has actually a demarcation region formed in the shape of stripes. In the case that such a mask is utilized, the exposure of each division is carried out, while scanning electrically the inside of one zonal division partitioned by the stripe-shaped demarcation region, with the electron beam, in the direction of the length.
In the SAL type electron-beam exposure method as described above, the proximity effect correction is performed as follows. Firstly, some of electrons that are scattered by the scatterer on a scattering membrane mask are selectively allowed to pass through an annular opening which is set in a limiting aperture section disposed in the position or the vicinity of the cross-over, and then these scattered electrons allowed to pass are defocused to about the back-scattering range by spherical aberration of a projection lens and used as a correction exposure (GHOST exposure) beam to irradiate the wafer.
A schematic view of an optical system to explain the proximity effect correction in the SAL type method is shown in FIG. 7. Image-forming electrons passing through a mask 201 are focused by a first projection lens 202, and then pass through a central opening in a limiting aperture section 203, that is disposed in the cross-over plane or the back-focal plane, and subsequently form an image on a resist 206 on a wafer 205 by a second projection lens 204. The resist 206 in FIG. 7 is a negative one, of which an irradiated portion is to remain, and showing the form after development for illustration.
Meanwhile, most of electrons scattered by the mask 201 are blocked by the limiting aperture 203 and only a small part of the electrons pass through the central opening and an annular opening that surrounds the central opening. These mask-scattered electrons passed therethrough are defocused to about a back-scattering range xcex2b by the spherical aberration of the second projection lens 204, and distributed over the wafer as a correction exposure (GHOST exposure) beam. The central and the annular openings are concentrically disposed.
The intensity of the correction beam and, therefore, the correction dose that is proportional to the intensity is normally controlled by the area of the annular opening, and the range of defocusing, by the distance of the annular opening from the center of the limiting aperture or the radius of the opening. Since the opening area of the annular opening is larger than that of the central opening, the proximity effect correction, in practice, mostly depends on the scattered electrons passing through the annular opening. Further, in the actual design of a limiting aperture for exposure, as the back-scattering range largely depends on the wafer material and the accelerating voltage, under the same conditions of the wafer material and accelerating voltage, the position of an annular opening with respect to the center of the limiting aperture is set constant (the degree of defocusing is constant). Since the optimum correction dose depends on the substrate material, namely, the back-scattering coefficient xcex7, the adjustment of the correction dose is made by changing the width of the annular opening (the opening area) according to the underlying substrate.
Next, referring to FIGS. 8 and 9, the basic principle of proximity effect correction in the afore-mentioned optical system shown in FIG. 7 is described.
FIG. 8(a) shows a scattering membrane mask, and referential numerals 301 and 302 indicate a membrane and a scatterer layer, respectively. FIG. 8(b) shows a distribution of energy deposition in the resist on the wafer when using a limiting aperture without an annular opening and providing no correction beam, in other words, when making no proximity effect correction, while FIG. 8(c) shows a distribution of energy deposition when using a limiting aperture with an annular opening and providing a correction beam, in other words, when making proximity effect correction. In the drawings, xcex2b is a back-scattering range. Assuming the energy of the forward-scattering electrons is 1, the back-scattering electrons have an energy corresponding to the back-scattering coefficient xcex7, and the correction dose ration xcex4 required in this instance is given by xcex7/(1+xcex7). The energy of the back-scattering electrons can be obtained as the product of the pattern density and the back-scattering coefficient.
By defocusing the correction beam to about the back-scattering range xcex2b or L, the deposition energy, which has been lowered near the borderline, as seen in FIG. 8(b), can be brought to a constant level, as seen in FIG. 8(c). This results in an improvement of linewidth accuracy of the pattern.
In FIG. 9, the scattering membrane mask in FIG. 8(a) is replaced with a mask on which there is formed a line and space pattern (1/1), in other words, a pattern with a pattern density of 50%. As clearly seen in FIG. 9, even when the pattern density is changed, the proximity effect correction can be made in the same way.
However, the afore-mentioned proximity effect correction method for the SAL type electron-beam exposure method by G. P. Watson and others has a problem that regulation of the degree of defocusing and the amount of correction dose for correction exposure (GHOST exposure) is difficult.
The proximity effect correction method by G. P. Watson and others is a method in which the correction exposure is carried out concurrently with the pattern exposure and thereby the proximity effect correction is made. Because one exposure can accomplish both the pattern exposure and the proximity effect correction therein, this correction method has the advantage of a high throughput. Nevertheless, this method can only provide a high throughput and an excellent proximity effect correction when pattern projections are performed repeatedly, using one and the same mask and also applying electron-beam exposure to a substrate of the identical kind for exposure.
Apart from accelerating voltage, the degree of the proximity effect varies with the pattern density, the material of a substrate for exposure such as a wafer, the thickness of the resist film or the like. Therefore, when a mask having different patterns is utilized, when a substrate for exposure that is made of a different material is employed or when a resist layer is formed with a different thickness in any step of lithography, it is necessary to adjust the correction dose and the degree of defocusing in order to make the proximity effect correction appropriate to the particular mask, the substrate for exposure and the film thickness of the resist. Also when the thickness of the electron-beam scatterers varies with the mask, the scattering angles of the scattered electrons are changed and, accordingly, the correction dose is changed so that the correction dose must be adjusted all over again.
Because the adjustment of the correction dose is made by changing the radius and the width (the area) of the annular opening in the limiting aperture section 203, another limiting aperture section having a different annular opening must be in effect prepared separately. Moreover, to make exchange of the limiting aperture section, the electron-beam exposure must be suspended once and, relinquishing a vacuum condition, the inside of the optical system must be made open to the air beforehand. In short, the conventional method as described above has a problem that, if the optimum proximity effect correction suitable to the mask patterns and the substrate for exposure is to be achieved, a marked lowering of throughput must turn up.
Further, in practice, if the thickness of the scatterer layer formed over the membrane is varied within the mask plane in a fabricated scattering membrane mask, the scattering angles of electrons are changed in response to that. This alters the correction dose and makes the proximity effect correction insufficient. Consequently, in manufacturing a mask, uniformity of the highest standard is demanded for the film thickness throughout a mask plane but fabrication of a scattering membrane mask having such a highly uniform film thickness is not an easy task, resulting in a problem of lower yield and higher cost.
Further, apart from pattern density, within patterns for one chip or for a portion of several segments thereof, the extent of the proximity effect may vary with the underlying pattern. For example, when an underlying pattern of an interconnection or the like is formed of a heavy metal such as tungsten or the like on an underlying layer of the resist layer that is placed on the wafer surface, the incident electrons are reflected or back-scattered by that underlying pattern and, consequently, the extents of the proximity effect may become very much different between the resist region over the region where no underlying pattern is formed and the resist region over the underlying pattern formation region. In the afore-mentioned conventional correction method, correction exposure cannot be adjusted partially corresponding to the partially changed proximity effect within patterns for one chip or for a portion of several segments thereof.
An object of the present invention is to provide a method of electron-beam exposure wherein regulation of the correction exposure for the proximity effect correction can be made easily and excellent linewidth accuracy can be attained, and a mask as well as an electron-beam exposure system used therein.
The present invention relates to an electron-beam exposure method of segmented mask-pattern transfer type wherein a prescribed pattern is segmented into a plurality of divisions so as to form a segmented pattern in every said division and exposure is made through every said division one after another, whereby the projection of the whole of said prescribed pattern is accomplished; which comprises the steps of:
carrying out the exposure through every said division and transcribing a segmented pattern thereon one after another, and
carrying out the correction exposure for every projection region of said segmented patterns one after another with a defocused beam of the inverse pattern of respective said segmented patterns, and thereby the proximity effect caused by the pattern exposure is corrected.
Further, the present invention relates to the electron-beam exposure method of segmented mask-pattern transfer type as set forth above; which comprises the steps of:
carrying out the exposure through every said division and transcribing a segmented pattern thereon one after another, while using a mask having, on one and the same substrate, a group of segmented patterns which are formed by segmenting the prescribed pattern into a plurality of divisions so as to form a segmented pattern in every said division and a group of inverse patterns of these segmented patterns; and
carrying out the correction exposure for every projection region of said segmented patterns one after another with a defocused beam of the inverse pattern of respective said segmented patterns, and thereby the proximity effect caused by-the pattern exposure is corrected.
Further, the present invention relates to the electron-beam exposure method of segmented mask-pattern transfer type as set forth above; which comprises the steps of:
carrying out the exposure through every said division and transcribing a segmented pattern thereon one after another, while using a first mask having a group of segmented patterns which are formed by segmenting the prescribed pattern into a plurality of divisions so as to form a segmented pattern in every said division; and
carrying out the correction exposure for every projection region of said segmented patterns one after another with a defocused beam of the inverse pattern of respective said segmented patterns, while using a second mask having a group of inverse patterns of said segmented patterns, and thereby the proximity effect caused by the pattern exposure is corrected.
Further, the present invention relates to the electron-beam exposure method of segmented mask-pattern transfer type as set forth above, wherein a stencil mask is used as the first mask and a scattering membrane mask is used as the second mask.
Further, the present invention relates to a mask for the electron-beam exposure that is used in the electron-beam exposure method as set forth above, which has, on one and the same substrate, a group of segmented patterns which are formed by segmenting the prescribed pattern into a plurality of divisions so as to form a segmented pattern in every said division and a group of inverse patterns of these segmented patterns.
Further, the present invention relates to an electron-beam exposure system having:
a structure which, with the mask as set forth above being disposed, can transfer said prescribed pattern by carrying out the exposure through the segmented pattern in every division one after another, and can apply the exposure to every projection region of the segmented pattern with the beam of the inverse pattern thereof by carrying out the exposure through the inverse pattern in every division one after another; and
a structure which can defocus the beam of the inverse pattern every time the beam of the inverse pattern is used for exposure.
In the present invention, even when a mask is replaced with another one having different patterns or a substrate for exposure is changed with another one having a different back-scattering coefficient and then the electron-beam exposure is performed, the correction dose can be easily adjusted to the pattern density or the back-scattering coefficient thereat, by simply changing the irradiation time in carrying out the correction exposure through the inverse pattern in every division one after another. Further, when electron-beam exposure is performed with a substrate for exposure being replaced by another one made of a material having a different back-scattering range, the degree of defocusing can be adjusted to the back-scattering range by a dynamic focus lens in carrying out the correction exposure through the inverse pattern in every division one after another. In this way, the present invention can make the optimum proximity effect correction in accordance with the type of the mask and the substrate for exposure without lowering the throughput and, thus, can obtain an excellent linewidth accuracy.
Further, in the present invention, even within prescribed patterns for on chip or for a portion of several segments thereof, in carrying out the correction exposure through the inverse pattern in every division one after another, the correction dose can be locally regulated according to the locally varied proximity effect by adjusting the irradiation time of every shot to the extent of back-scattering which depends on the pattern density and the effects of the underlying pattern. Further, in the present invention, even within prescribed patterns for on chip or for a portion of several segments thereof, in carrying out the correction exposure through the inverse pattern in every division one after another, the degree of defocusing of the inverse pattern beam can be locally regulated according to the locally varied proximity effect by adjusting a dynamic focus lens for every shot to the effects of the underlying pattern which has a different back-scattering range from the one of the substrate. In this way, the present invention can provide, within the prescribed pattern that is to be formed, the optimum correction exposures that vary with the location in response to the locally varied proximity effect, without lowering the throughput and, thus, can obtain excellent linewidth accuracy.
Further, the present invention can make the proximity effect correction using one and the same mask at any accelerating voltage, because the amount of blur of the correction exposure beam can be regulated by the dynamic focus lens, even if the back-scattering range changes with the accelerating voltage of the exposure system.
In the present invention, because both the primary patterns and inverse patterns are formed on the mask, no matter whether the resist employed is negative type or the positive type, excellent linewidth accuracy can be obtained by the same operations, using the same mask.
Further, in the present invention, the scattered electrons do not take part in the correction exposure so that uniformity of high standard is not required for the scatterer layer of the scattering membrane mask. As a result, the scattering membrane mask can be manufactured easily with low cost.
Further, in the present invention, since data for inverse pattern can be readily obtained, in manufacturing a mask, within a short time only by reversing tone of the pattern data in the CAD (Computer Aided Design) data, a lengthy data processing time that is hitherto required for the proximity effect correction can be drastically cut down. Further, when the positive patterns and the inverse patterns are formed on separate masks, if the negative resist and positive resist are employed for respective masks in fabricating masks, even the tone reversal of the pattern data described above becomes unnecessary. In this case, therefore, any data processing is not required for the proximity effect correction and, thus, pretreatment time for the proximity effect correction can be further reduced greatly.
Further, in the present invention, by employing a stencil mask and a scattering membrane mask as the masks for the prescribed pattern and the inverse pattern, respectively, masks can be fabricated easily without trouble even if patterns that may bring about the doughnut problem or leaf problem at the time of formation of inverse pattern are formed, and besides this makes possible to carry out the pattern exposure with high resolution.